This invention relates generally to glass-ceramic sensors and transducers and in particular to methods for making the glass-ceramic components of such sensors.
Ohnesorge et al, U.S. Pat. No. 4,422,335 discloses a pressure transducer made using quartz plates and Bernot, U.S. Pat. No. 5,189,591 discloses a pressure transducer using alumino-silicate glass plates. Whether glass or quartz, these sensors have generally been mounted to headers made of aluminum or stainless steel. Besides being expensive to machine, another disadvantage to these aluminum and stainless steel headers is that it is difficult to hermetically seal the electrical connections to these sensors. Without a hermetic seal, liquid and gas contaminants in the sensing medium can enter and ruin the sensor. As a result these types of sensors can only be used where the pressure medium is contaminant free such as dry gasses.
A further advance in sensor technology is disclosed in Bernot et al, U.S. patent application Ser. No. 08/986,253, filed Dec. 5, 1997 now U.S. Pat. No. 6,058,780, entitled, (as amended), xe2x80x9cCapacitive Pressure Having a Ceramic Basexe2x80x9d which is assigned to the assignee of this application. This application discloses a sensor in which the prior art aluminum or stainless steel headers, (also referred to as bases), are replaced with a ceramic header and a cover that is either ceramic or metallic. The cover and header are sealed together with glass frit to define an interior chamber. A capacitive pressure sensor with alumino-silicate glass plates is mounted within the chamber.
Using a ceramic or glass-ceramic has the advantage that its coefficient of thermal expansion can be tailored through composition and firing condition modification to closely match the coefficient of thermal expansion of the alumino-silicate sensor glass plates. Further strengthening and toughening of the glass-ceramic can be achieved with the addition of second phase ceramic fillers. Several glass-ceramic materials have been used to fabricate prototype headers by machining. However, machining is a relatively expensive process.
Accordingly, there is a need for a more economical method of fabricating the sensor header and cover.
An object of the present invention is to provide a method for fabricating a sensor header or cover from a glass-ceramic.
The present invention accomplishes this object by providing a dry pressing process and a slip casting process for fabricating these glass-ceramic components. Both processes start with a powder composition of SiO2 (42-59 wt. %), Al2O3 (17-35 wt. %), MgO (2-25 wt. %) and also BaO (0-20 wt. %), TiO2 (0-12 wt. %), ZnO (0-10 wt. %), CaO (0-10 wt. %), B2O3 (0-5 wt %), P2O5 (0-5 wt. %), ZrO2 (0-5 wt. %), alkali oxides such as K2O, Na2O (0-3 wt. %) and other small amounts (0-1 wt. %) of additives such as Fe2O3, V2O5, As2O3. Some of the ingredients (e.g. Mg, Ba, Ca) can also be present in the form of fluoride, e.g., MgF2, BaF2, CaF2, to up to 10 wt. %. Additional ceramic additives up to 30 wt. % which are as second phase (e.g., Al2O3, Y2O3 doped ZrO2, SiO2, mullite, zircon, carbides, nitrides, and a combination of the above), are not part of the glass composition, but are introduced to enhance both the mechanical strength and toughness and to further modify the coefficient of thermal expansion. In the dry pressing process the powder is milled, blended with a binder and then subjected to a drying/granulation process. A die cavity of a desired shape is then filled with the powder. Pressure is then applied to die for a period of time. After the pressure is removed, the green ceramic part is ejected and then sintered at high temperatures to become) a dense, finished ceramic cover or header.
In the slip casting process, the powder is milled and dispersed in water. The slip is then poured or pumped into a permeable casting mold of a desired shape made of gypsum. When either the liquid has been completely sucked away by the mold or the desired depositing thickness has been reached, the cast green ceramic part is removed from the mold after partial drying. After completely drying under controlled conditions, the green ceramic part can be sintered at high temperatures to become a finished ceramic cover or header.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.